Mimas or Saturn I is the eighth moon of Saturn in order of distance, the sixth moon of Saturn to be discovered, and the seventh heaviest.[8] It is also the smallest known body in the solar system that maintains a hydrostatic equilibrium shape.

Discovery and naming

Sir John Herschel, his son, suggested the current names of the seven largest satellites of Saturn, including Mimas. Titan received a generic name, and the other six received names of the Titans of mythology. Mimas was a giant who fell in battle against either Hephaestus or Ares in the Titan-Olympian War.[10]

Orbital characteristics

Mimas is in a somewhat eccentric orbit around Saturn. Its semimajor axis is 185,520 km, and thus it is outside the main body of the rings but within the E ring. Its sidereal day is about 22.5 hours.

Rotational characteristics

Physical characteristics

Mimas is slightly more dense than water, and thus is probably composed mostly of water ice.[2][11][12] The tidal interaction between it and Saturn causes it to assume an ellipsoidal shape. Its radial semi-axis is 209 km.[4]

Surface

The surface of Mimas is covered with multiple impact craters. Most of these are very deep, apparently because Mimas is not large enough for its self-gravity to relax the surface.[13]

The largest and most prominent impact crater is named Herschel, after the discoverer.[8] It measures 130 km in diameter and 10 km deep. Its walls rise 5 km high, and its central mountain rises 6 km above the floor.[11][2][12] The Cassini orbiter has photographed apparent fracture marks on the side of Mimas opposite Herschel. This last has caused debate among astronomers as to how great an impact a body can suffer without shattering.[2][12] In fact, some astronomers even speculate that a much larger body once occupied Mimas' place in the Saturnian system, and suffered a shattering impact. The fragments then reassembled themselves to form Mimas.[12]

The remaining craters vary in diameter from 20 km to 40 km.[2] Curiously, the south pole region has few craters larger than 20 km in diameter.[2]

Observation and Exploration

Voyager 1 was the first to visit and photograph Mimas. In the process, Voyager 1 discovered the Herschel crater.

The Cassini orbiter has taken many more images of much higher quality. Its closest rendezvous with Mimas took place on August 2, 2005. No further rendezvous are currently planned.

Gallery

Mercator map of Mimas, annotated with plans for detailed coverage by Cassini